In "On His Blindness," Milton wants to emphasize the fact that those who serve God best are the ones who allow their fates to be yoked to his. In metaphorical terms, it is as if we were all part of a team of oxen controlled by a gigantic divine harness. In the Bible, that is exactly what a yoke was—a harness used for driving oxen.

However, Milton, in the guise of personified Patience, realizes that this may sound as if God is some kind of slave driver, exercising brutal, dictatorial control over us. So he's at pains to point out that the burden imposed upon us by the Almighty is a "mild yoke"—that is to say, not a very heavy burden to bear.

Milton fervently believes that if we can see our relationship to God in this way, we'll be much better able to deal with life's many trials and tribulations. So long as we realize that God is always in control, that we are always under his "mild yoke," as it were, we will be more likely to accept whatever misfortunes may befall us, such as the speaker's blindness, which in those days was an even greater misfortune than it is today.